The knowledge and control of radial growth of turbo-machinery components has long been a stumbling block on the way to achieving higher efficiency and stability levels demanded by the designers of gas turbine engines, pumps and compressors. This undesirable situation is driven in part by lack of reliable, accurate and affordable sensors for measuring radial growth. Alternatively, the radial growth can be computed using a mathematical model that relates growth to various turbomachine measured and otherwise obtained parameters. Numerous attempts were made in the past to devise such an algorithm. However, none of the known algorithms delivered required steady state and transient accuracy, ability to calibrate the equations to high fidelity data and formulation suitable for implementation in a digital computer.
Imperfect control of the clearance between a turbine engine fan blade and case can result in either the clearance being too loose or the clearance being too tight resulting in excessive rubs. In either instance, imperfect clearance results in loss of performance (e.g. engine efficiency, thrust) and/or violation of the engine operating limits (e.g. exhaust gas temperature overshoot) and/or reduced compressor stability. Standard practice has been to design a clearance control system to prefer loose clearance over tight clearance which may also result in damage to the blades and case. Some engines such as, for example, the PW4000 use an open loop clearance control system that sacrifices significant performance in comparison with a “perfect” clearance control system. Other engines such as, for example, the V2500 use a closed loop system that relies on crudely modeled clearances and therefore sacrifices less performance, but still falls short of ideal clearance control.
Improved accuracy and reliability in estimating tip clearances will also enable the clearance control system to be active during those parts of an airplane mission that are more likely to experience abrupt changes in operating conditions. For example, a typical active clearance control system is traditionally deactivated during airplane takeoff where tip clearances are particularly hard to predict due to rapidly changing engine operating conditions. This approach worked well in the past for the cases where takeoff constituted a relatively small portion of the overall airplane mission and the engine stability margins were conservatively high. In contrast, takeoff fuel economy gains importance for the engines designed for short haul aircraft applications such as, for example, PW6000 designed for A318 application. The ability to deploy active clearance control during takeoff also increases the exhaust gas temperature margin which otherwise diminishes with increased clearance, and helps to avoid clearance induced stability loss. Thus, it is desirable to further improve clearance control accuracy to, in turn, improve engine performance while maintaining all operating limits, compressor stability and ensuring reliable rub-free operation throughout the airplane mission.
The principal difficulty in modeling clearances for a closed loop system resides in modeling the thermal growths of the engine components, not in modeling the mechanical strains which are relatively easy to calculate. Thermal growths are far more difficult to model because the physical configurations of the engine components and the multiple time varying influences to which those components are subjected (i.e., throttle transients, multiple fluid streams of different and time varying temperatures, flow rates, etc.) complicate the problem of modeling the heat transfer and energy storage phenomenon.
For instance, engine components each experience thermal growth at their own respective pace due to their location with the engine housing, varying operating conditions including temperatures, shaft speeds, fluid stream exposure. As a result one component may experience a greater amount of thermal growth than another component such that one area of the gas turbine engine may experience a greater amount of thermal growth than another area. In turn, the internal wall of the engine housing opposite these varying areas of thermal growth on the gas turbine engine will also experience varying amounts of thermal growth due to the inconsistent heat transfer occurring between engine components. As a result, one area of the internal wall of the engine housing may exhibit a greater amount of thermal growth and correspondingly a smaller clearance as opposed to another area of the engine housing. At that point the obstacles pertaining to modeling the heat transfer and energy storage phenomenon of a gas turbine engine become more apparent.
To that end there is a need for systems and methods directed to controlling thermal growth, maintaining clearance control and monitoring the health of turbomachinery applications.